How could one design a secure and “self-destructing” email?

Question

As most of you know, email is very insecure. Even with a SSL-secured connection between the client and the server that sends an email, the message itself will be in plaintex

Answer 1

If HTML format is used, you can have the message reference assets that you can remove at a later date. If the message is open at a later date, the user should see broken links..